1. Field of the Invention
This invention relates to an organic positive temperature coefficient thermistor that is used as a temperature sensor or overcurrent-protecting element, and has positive temperature coefficient (PTC) of resistivity characteristics that its resistance value increases with increasing temperature.
2. Background Art
An organic positive temperature coefficient thermistor having conductive particles dispersed in a crystalline thermoplastic polymer is well known in the art, as disclosed in U.S. Pat. Nos. 3,243,753 and 3,351,882. The increase in the resistance value is believed to be due to the expansion of the crystalline polymer upon melting, which in turn cleaves a current-carrying path formed by the conductive fine particles.
An organic positive temperature coefficient thermistor can be used as a self-regulated heater, an overcurrent-protecting element, and a temperature sensor. Requirements for those are that the resistance value is sufficiently low at room temperature in a non-operating state, the rate of change between the room-temperature resistance value and the resistance value in operation is sufficiently large, and the resistance value change upon repetitive operations is reduced.
To meet such requirements, it has been proposed to incorporate a low-molecular weight organic compound such as wax in a thermoplastic polymer matrix serving as a binder. Such organic positive temperature coefficient thermistors, for instance, include a polyisobutylene/paraffin wax/carbon black system (F. Bueche, J. Appl. Phys., 44, 532, 1973), a styrene-butadiene rubber/paraffin wax/carbon black system (F. Bueche, J. Polymer Sci., 11, 1319, 1973), and a low-density polyethylene/paraffin wax/carbon black system (K. Ohe et al., Jpn. J. Appl. Phys., 10, 99, 1971). Self-regulated heaters, current-limiting elements, and other devices comprising an organic positive temperature coefficient thermistor using a low-molecular organic compound are also disclosed in JP-B 62-16523, JP-B 7-109786 and JP-B 7-48396, and JP-A 62-51184, JP-A 62-51185, JP-A 62-51186, JP-A 62-51187, JP-A 1-231284, JP-A 3-132001, JP-A 9-27383 and JP-A 9-69410. In these cases, the increase in the resistance value is believed to be due to the melting of the low-molecular organic compound.
One advantage associated with the use of the low-molecular weight organic compound is that there is a sharp rise in the resistance increase with increasing temperature because the low-molecular weight organic compound is generally higher in crystallinity than a polymer. A polymer, because of being easily put into an over-cooled state, shows a hysteresis where the temperature at which there is a resistance decrease with decreasing temperature is usually lower than the temperature at which there is a resistance increase with increasing temperature. The use of low-molecular weight organic compound can hold down this hysteresis. Further, a combination of low-molecular weight organic compounds having various melting points makes it possible to easily control the temperature (operating temperature) at which there is a resistance increase. A melting point of a polymer is susceptible to molecular weight and crystallinity, and its copolymerization with a comonomer, resulting in a change of operating temperature. Since this is accompanied by a variation in the crystal state, sufficient PTC characteristics are not always obtained. This is particularly true of the case where the operating temperature is set below 100xc2x0 C.
However, the organic PTC thermistors disclosed in the above references, in which carbon black or graphite is used as the conductive particles, fail to find a good compromise between a high initial or room temperature resistance and a large resistance change rate. For instance, Jpn. J. Appl. Phys., vol. 10, P.99, 1971 shows an example wherein the resistivity (xcexa9-cm) increased to 108 xcexa9-cm. However, its resistivity at room temperature is as high as 104 xcexa9-cm, indicating that the device is impractical for an overcurrent-protecting element or temperature sensor in particular. The other references show resistance (xcexa9) or resistivity (xcexa9-cm) increases in the range between 101 times or lower and about 104 times, while the room-temperature resistance is not fully low.
JP-A 5-47503 discloses an organic PTC thermistor comprising a crystalline polymer and conductive particles having spiky protuberances. Also, U.S. Pat. No. 5,378,407 discloses an organic PTC thermistor comprising filamentary nickel powders having spiky protuberances, and a crystalline polyolefin, olefin copolymer or fluoropolymer.
In these thermistors, the tradeoff between low initial resistance and a large resistance change is improved. However, they are still insufficient in terms of hysteresis, because low-molecular weight organic compounds are not used as the operating substance, and so are unsuitable for applications such as temperature sensors. They undesirably show negative temperature coefficient (NTC) of resistivity characteristics that the resistance value decreases with increasing temperature, when they are further heated after the resistance has once increased during operation. The use of low-molecular weight organic compounds is taught nowhere in the above references. In addition, most of these thermistors have an operating temperature of higher than 100xc2x0 C. Although some thermistors have an operating temperature in the range of 60 to 70xc2x0 C., they are impractical because their performance becomes unstable upon repetitive operations.
In JP-A 11-168005, the inventors proposed an organic PTC thermistor comprising a thermoplastic polymer matrix, a low-molecular weight organic compound, and conductive particles having spiky protuberances. This thermistor has a sufficiently low room-temperature resistivity of 8xc3x9710xe2x88x922 xcexa9-cm or less, a rate of resistance change of eleven orders of magnitude or greater between an operating state and a non-operating state, and a reduced temperature vs. resistance curve hysteresis. In addition, the operating temperature is 40 to 100xc2x0 C. By virtue of a good compromise between low room-temperature resistivity and a large resistance change as well as a low operating temperature, this thermistor is best suited as an overcurrent or overheat-protecting element for a secondary battery.
However, this thermistor was found to be insufficient in performance stability, which is probably ascribable to the low melting point and low melt viscosity of the low-molecular weight organic compound. The probable cause is that melting and solidification alternately occur during the operation and restoration of the device, which invites changes in the crystalline and dispersion states. Especially in a hot humid accelerated test and cyclic load test, outstanding increases of resistance developed.
The above-referred U.S. Pat. No. 5,378,407 describes that the crystalline polyolefin, olefin copolymer or fluoropolymer may be further blended with an elastomer, amorphous thermoplastic polymer or another crystalline polymer. However, specific examples and benefits of this blending are not disclosed. JP-A 54-16697 discloses a PTC composition comprising a polymer component of at least two crystalline thermoplastic polymers having separate melting points, and an elastomer, in which only carbon black is used as the conductive particles. This composition fails to exhibit both a low room-temperature resistance and a large resistance change. No reference is made to the reliability against thermal shock test or the use of low-molecular weight organic compounds.
Additionally, JP-A 63-279589, JP-A 5-266974, JP-A 8-138439, JP-A 8-316005, and JP-A 10-334733 disclose PTC compositions comprising elastomers. No reference is made to conductive particles having spiky protuberances and low-molecular weight organic compounds. Therefore, these compositions fail to exhibit both a low room-temperature resistance and a large resistance change.
An object of the invention is to provide an organic PTC thermistor having improved reliability and performance stability.
The invention provides an organic positive temperature coefficient (PTC) thermistor comprising a matrix of at least two polymers, a low-molecular weight organic compound, and conductive particles having spiky protuberances, wherein the matrix contains a thermoplastic elastomer.
In one preferred embodiment, the matrix contains 2 to 60% by weight of the thermoplastic elastomer, based on the entire weight of the matrix. The low-molecular weight organic compound preferably has a melting point of 40xc2x0 C. to 200xc2x0 C. The low-molecular weight organic compound is preferably segregated in the matrix. Desirably the conductive particles are interconnected in chain-like network.
In a typical application, the thermistor constitutes a protective circuit device for a secondary battery.
The organic PTC thermistor of the invention comprises a polymer matrix, a low-molecular weight organic compound, and conductive particles having spiky protuberances. The spiky shape of protuberances on the conductive particles enables a tunnel current to pass readily through the thermistor, and makes it possible to obtain a room temperature resistance lower than would be possible with spherical conductive particles. A greater spacing between protuberant particles than between spherical particles allows for a large resistance change during operation of the thermistor.
In the present invention, the low-molecular weight organic compound is incorporated in the polymer matrix so that the resistance is increased by expansion due to melting of the low-molecular weight organic compound. Accordingly, the temperature vs. resistance curve hysteresis can be reduced as compared with the operation based on the melting of crystalline polymer. Control of the operating temperature becomes easy when low-molecular weight organic compounds having various melting points are used.
The polymer matrix used herein is composed of a plurality of polymers. Using a thermoplastic elastomer as part of the polymer matrix, the device is further improved in long-term durability. The polymer matrix is effective for preventing the device from being deformed when the low-molecular weight organic compound having a low melt viscosity melts during operation. Using a thermoplastic elastomer as one component of the polymer matrix, the device is improved in thermal shock resistance and maintains a low room-temperature resistance and a large resistance change in a stable manner over a long term.